U.S. patent application number 10/388563 was filed with the patent office on 2004-01-01 for method for developing in hybrid developing apparatus.
Invention is credited to Nishimura, Toshinori, Ozawa, Yoshio, Sakata, Shoichi.
Application Number | 20040002015 10/388563 |
Document ID | / |
Family ID | 29781767 |
Filed Date | 2004-01-01 |
United States Patent
Application |
20040002015 |
Kind Code |
A1 |
Ozawa, Yoshio ; et
al. |
January 1, 2004 |
Method for developing in hybrid developing apparatus
Abstract
To avoid developing ghost and selective developing without
complicating a developing apparatus and to supply surly charged
toner to a developing roll while unevenness of images is not
generated to obtain stable images for a long time even when
continuous printing, a method for developing in a hybrid developing
apparatus is provided, which is characterized in that a data amount
of each image, which an image forming apparatus prints and a number
of paper sheets passed are detected and that an interval of the
action of recovering and exchanging a toner layer on the developing
roll to the magnetic roll after printing (between paper sheets) is
variably set in accordance with a printing coefficient determined
by the data amount and the number of paper sheets passed and
further, a method for developing in a color tandem-type image
forming apparatus provided with a plurality of the hybrid
developing apparatuses for all different colors of toner, which are
disposed along the moving direction of a transfer medium of images
is provided, wherein after toner layers on the developing rolls of
the all developing apparatuses are peeled off toward the magnetic
rolls after developing, a thickness of the toner layer of the each
developing apparatus is variably controlled at the beginning of
developing in accordance with the image consistency of the
developing apparatus having a highest image consistency among image
data of toner colors.
Inventors: |
Ozawa, Yoshio; (Watarai-gun,
JP) ; Sakata, Shoichi; (Watarai-gun, JP) ;
Nishimura, Toshinori; (Watarai-gun, JP) |
Correspondence
Address: |
SCHULTE ROTH & ZABEL LLP
ATTN: JOEL E. LUTZKER
919 THIRD AVENUE
NEW YORK
NY
10022
US
|
Family ID: |
29781767 |
Appl. No.: |
10/388563 |
Filed: |
March 14, 2003 |
Current U.S.
Class: |
430/122.1 ;
399/270; 399/274; 399/53 |
Current CPC
Class: |
G03G 15/0907
20130101 |
Class at
Publication: |
430/122 ;
399/274; 399/270; 399/53 |
International
Class: |
G03G 013/09; G03G
015/09 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 15, 2002 |
JP |
2002-072666 |
Mar 26, 2002 |
JP |
2002-086249 |
Claims
What is claimed is
1. A method for developing in a hybrid developing apparatus having
a magnetic roll for transferring two-component developer material,
which electrostatically charges developer material consisting of
carrier and toner magnetically held thereon, and a developing roll,
which forms a toner thin layer on its surface by transferring the
toner utilizing a bias for transferring and magnetic brushes of the
magnetic roll wherein latent images on a latent image bearing body
are developed with the toner by applying developing bias to nearest
approached regions (developing regions) between the developing roll
and the latent image bearing body, characterized in that amount of
data of each image, which an image forming apparatus prints and a
number of paper sheets passed are detected and that an interval of
the action of recovering and exchanging a toner layer on the
developing roll to the magnetic roll after printing (between paper
sheets) is variably set in accordance with a printing coefficient
determined by the data amount and the number of paper sheets
passed.
2. A method for developing in a hybrid developing apparatus
according to claim 1, wherein a rotational rate of the developing
roll is 1.1 times or more that of the latent image bearing body and
a rotational rate of the developing roll is from one to two times
that of the magnetic roll.
3. A method for developing in a hybrid developing apparatus
according to claim 1, wherein changing rates of the volume average
particle size of the toner on the developing roll is controlled
within 15% by recovering and exchanging a toner layer on the
developing roll to the magnetic roll.
4. A method for developing in a hybrid developing apparatus
according to claim 1, wherein when the toner layer on the
developing roll is recovered and exchanged to the magnetic roll, a
toner amount on the developing roll is changed within the range
from 0.5 to 1.7 mg/cm.sup.2 in accordance with amount of data of
each image.
5. A method for developing in a hybrid developing apparatus
according to claim 1, wherein an amount of toner charge on the
developing roll at the beginning of developing is controlled within
the range of from 5 to 20 .mu.C/g by recovering and exchanging the
toner layer on the developing roll to the magnetic roll.
6. A method for developing in a hybrid developing apparatus
according to claim 1, wherein the surface of the developing roll
consists of an approximately uniform electroconductive sleeve
having a conductivity of 10.sup.6 .andgate..multidot.cm.sup.3 or
less to which a bias is applied, and the applied bias is acted
between the developing roll and the latent image bearing body and
between the developing roll and the magnetic roll so that a toner
layer on the developing roll is recovered and exchanged to the
magnetic roll after printing (between paper sheets).
7. A method for developing in a color tandem-type image forming
apparatus provided with a plurality of hybrid developing
apparatuses defined in claim 1 for all different colors of toner,
which are disposed along the moving direction of a transfer medium
of images, wherein after toner layers on the developing rolls of
the all developing apparatuses are peeled off toward the magnetic
rolls after developing, a thickness of the toner layer of the each
developing apparatus is variably controlled at the beginning of
developing in accordance with the image consistency of the
developing apparatus having a highest image consistency among image
data of toner colors.
8. A method for developing in a color tandem-type image forming
apparatus according to claim 7, wherein a toner layer thickness at
the beginning of developing is {fraction (1/10)}-{fraction (1/4)}
of the gap between a developing roll and a latent image bearing
body.
9. A method for developing in a color tandem-type image forming
apparatus according to claim 7, wherein a toner amount of the toner
layer on the developing roll controlled variably at the beginning
of developing is controlled to the range within from 0.5 to 1.5
mg/cm.sup.2.
10. A method for developing in a color tandem-type image forming
apparatus according to claim 7, wherein an amount of toner charge
on the developing roll at the beginning of developing is controlled
to the range from 5 to 20 .mu.C/g.
11. A method for developing in a color tandem-type image forming
apparatus according to claim 7, wherein the developing roll rotates
1.5 to 3 times as rapidly as the latent image bearing body.
12. A method for developing in a color tandem-type image forming
apparatus according to claim 7, wherein a developing bias
consisting of a direct current bias and an alternate current bias
superposed thereon is applied onto an electro-conductive sleeve on
the surface of the developing roll, the sleeve having a volume
resistivity of 10.sup.6 50.OMEGA..multidot.cm.sup.3 or smaller, so
as to recover the toner layer on the developing roll to the
magnetic roll after developing.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to an image forming apparatus,
particularly a color tandem image forming apparatus utilizing
electrophotography such as a copying machine, a printer, a
facsimile and a complex machine of those, more specifically,
relates to an image forming apparatus having a hybrid developing
apparatus which develops a latent image to which an isolated
charged toner is caused to jump from a developing roll holding the
isolated charged toner on it using two components developer
material, in which non-magnetic toner is charged by means of
magnetic carrier.
[0003] 2. Description of the Related Art
[0004] Heretofore, there are kinds of developing methods in an
image forming apparatus utilizing electrophotography such as a
copying machine, a printer, a facsimile and a complex machine of
those. One is a two-components developer material method which uses
toner and magnetic carrier; another is one-component developer
material method which uses nonconductive toner or conductive toner.
Another one is a hybrid developing method which develops a latent
image twohich an isolated charged toner is caused to jump from a
developing roll holding the isolated charged toner on it using two
components developer material, in which non-magnetic toner is
charged by means of magnetic carrier.
[0005] A two parts developer material method has advantages of such
as excellent charging property of toner by carrier, long life of
toner as well as uniformity of solid image. On the other hand, it
has drawbacks of such as a large and complicated developing
apparatus, generation of toner scattering or carrier accompanying
as well as varying image quality depending on durability of
carrier. A one-component developer material method has advantages
of a compact developing apparatus as well as good dot
reproducibility while it has drawbacks of the poor durability of a
developing roll and a charge roller, which results in selective
developing or high maintenance cost of supplies and material for
developing apparatus. A hybrid developing method is a high rate
image forming method which has good dot reproducibility and a long
life, however it has heretofore such problems as generation of
developing ghost or toner scattering.
[0006] Though a hybrid developing method has been developed as one
of measures for a one-component non-contact developing, it has been
recently developed as a high rate developing method for a
developing method of superimposing multi-colors on one drum, in
which a plurality of color images are sequentially formed on one
latent image bearing body (a photosensitive body). Attention has
been paid to this method as an art of quality color images because
a color image with scarce color drift can be obtained by accurately
overlapping different colored layers of toner on a latent image
bearing body.
[0007] However, since developing apparatuses of a number of colors
used should be disposed around the latent image bearing body in a
developing method of superimposing multi-colors on one drum, the
latent image bearing body becomes large in size so as to prevent
from designing a compact image forming apparatus. Hence, attention
has been paid to a tandem method in which a plurality of
electro-photographic process units corresponding to colored toners
used are aligned and colored images are formed synchronized with
transferring of a transfer member so as to overlapping the colored
images on the transfer member. However, this method has a drawback
that the image forming apparatus becomes large because
electro-photographic process units of kinds of colors should be
aligned, though it has an advantage in having a rapid rate of
printing. In order to improve measures against this problem, a
compact tandem image forming apparatus has been proposed, which is
provided with a small-sized image forming unit by narrowing a
distance between latent image bearing bodies.
[0008] The compact tandem image forming apparatus thus designed has
an advantage when it has a vertical developing apparatus in order
to minimize a size in width direction of the image forming unit.
That is, it is desirable regarding parts layout to dispose a
developing apparatus in the upper part of the latent image bearing
body. However, in case the developing apparatus is disposed in a
vertical manner in a conventional two-components developer material
method, flow of developer material, that is, the supply of
developer material from a developer mixer to the part adjacent to a
latent image bearing body is complicated so that down-sizing of the
apparatus is limited, carrier adheres inevitably to the latent
image bearing body and toner scatters unavoidably.
[0009] Though, as another method, a one-component developer
material method in which carrier is not used is proposed, it has a
drawback of enhancing color drift, which is the weak point of a
tandem type, on account of the torque fluctuation of a latent image
bearing body caused by contacting a developing roll to the latent
image bearing body. Meanwhile, with a method in which a latent
image bearing body is not touched, toner is charged by a charge
roll and the toner thickness on the developing roll is restricted
by an elastic restricting blade. Thus, additive for toner adheres
to the charge roll so that the charging ability is decreased or
toner adheres to the restricting blade so that toner thickness
becomes uneven, which results in image defects.
[0010] Hence, as a measure for resolving the problems, the
aforementioned hybrid developing method has attracted attention.
Though the hybrid developing method has the problems with regard to
generation of ghost and scattering of toner, the method has little
torque fluctuation owing to non-contacting between the developing
roll and the image bearing body as well as excellent dot
reproducibility so as to be able to provide a high-speed image
forming apparatus capable of a long life.
[0011] A prior art regarding a hybrid developing method is
disclosed in U.S. Pat. No. 3,866,574 which proposes a method
wherein a non-magnetic toner thin layer is formed on a donor roll
(developing roll) disposed untouchably toward a latent image
bearing body and the toner is caused to jump to the latent image on
the latent image bearing body by an alternative current electric
field. While, U.S. Pat. No. 3,929,098 shows a developing apparatus
which forms a toner layer by transferring the toner onto a donor
roll using a magnetic roll.
[0012] However, although these arts enable toner to form a thin
layer on a donor roll by means of two-components developer
material, a strong alternative current electric field is necessary
to separate toner from the donor when the electrostatic charge of
the toner is high. The strong alternative current electric field
disturbs the toner layer so that a problem concerning superimposing
multi-colors arises. In this regard, so called a powder cloud
developing method is disclosed in Japanese laid open patent
publication, No. JP1992-113474 wherein auxiliary electrodes
consisting of wires are provided between a donor roll and a latent
image bearing body applying a weak alternative current electric
field so as not to disturb the toner developed.
[0013] Theoretical aspect of the formation of toner layer on a
developing roll using two-component developer material was reported
by Toshiba Corporation in the Electrophotography Society Journal
19, 2 (1981) and the art is disclosed in Japanese laid-open patent
publication JP1984-121077.
[0014] The aforementioned prior arts have a tendency of decreasing
image consistency owing to selective developing as coarse particles
of the high developing natured toner are apt to selectively jump to
the latent image body and fine particles of the highly charged
toner remains on the developing roll sleeve. Further, the control
of charging toner is complicated so that a high surface potential
and a big developing electrical field need to be applied. In this
regard, if both toner consumption region and non-consumption region
are generated on the developing roll, toner adhering state and
toner potential vary on the surface of the developing roll. Thus,
as shown in FIG. 3, so called a hysteresis, a phenomenon in which a
part of previous developing image appears as a ghost is liable to
generate. In FIG. 3, 35 is a solid image consisting of a
rectangular black image and 36, 37 are half tone image succeeding
and broader than the solid image. When both toner consumption
region and non-consumption region are generated on the developing
roll, printing the half tone images 36, 37 after the solid image 35
generates the ghost shown in FIG. 3(b). Further in case high
consistent images are printed repeatedly, unevenness of image such
as mottle in image consistency is liable to occur so that a problem
in down sizing the apparatus arises.
[0015] In order to overcome this problem, Japanese laid-open patent
publication JP1999-231652 discloses a member for scraping the
residual toner on a developing roll and a recovery device for the
scraped toner, while Japanese laid-open patent publication
JP2000-81788 discloses an apparatus using an exclusive recovery
roll for assuring recovery of toner on the developing roll. However
these methods are necessary for complex structures enabling to make
practical for a compact electrophotographic apparatus. Further,
Japanese laid-open patent publication JP1995-128983 discloses an
apparatus wherein toner on a developing roll is recovered by
setting a broad half value region of magnetic density of a magnetic
roll as a measure for the hysteresis when using magnetic brushes.
Japanese laid-open patent publication JP1988-249164 discloses a
control method for a developing apparatus of tandem type wherein
degradation of developer material is prevented by bringing down the
operation of developing apparatus in an image forming part except
for the image forming part whereat a transfer process is performed.
In order to prevent big effect for image quality caused by
fluctuation of a toner concentration in two-component developer
material, Japanese laid-open patent publication JP1998-31366
discloses a developing apparatus of non-contact method for
two-component developer material wherein toner consumption is
predicted by image data by providing a plurality sets of developing
magnetic pole and a trimming magnetic pole for fixing toner
replenishing amount on a magnetic roll while a position of the
trimming magnetic pole is adjusted so as to be capable of supplying
a most appropriate toner amount from the actual toner amount on a
latent image bearing body and the predicted toner consuming amount
by providing a device for detecting a toner amount on the latent
image bearing body.
[0016] In a hybrid developing method, decreasing of image
consistency owing to selective developing or developing defect,
image degradation, developing ghost, toner scattering and sleeve
adhesion owing to leaving for a long lime while holding toner on a
developing roll are generated. In order to solve the aforementioned
problems, Japanese laid-open patent publication JP1994-67546,
JP1995-72733 and JP1995-92804 disclose an image forming apparatus
having a magnetic roll forming magnetic brushes by two-components
developer material, a donor roll (developing roll) holding a toner
thin layer supplied by the magnetic roll and electrodes disposed
between the donor roll and a latent image bearing body wherein bias
consisting of alternate current and direct current voltage are
applied to the electrodes, direct current bias is applied to the
developing roll and polarity reversible direct current bias by a
switch (JP1995-72733) or alternate current bias superposed on
direct current (JP1994-67546 and JP1995-92804) is applied to the
magnetic roll.
[0017] In a developing apparatus described in JP1994-67546 or
JP1995-72733, a toner thin layer is formed on the developing roll
with magnetic brushes generated on the magnetic roll with direct
current potential difference (JP1995-72733) or alternate current
bias (JP1994-67546 and JP1995-92804) between the magnetic roll and
the developing roll and further a latent image on the latent image
bearing body is developed by forming toner cloud near the
electrodes with an alternate current bias superposed on direct
current applied between the developing roll and the electrodes
while the toner on the developing roll is recovered by applying a
direct current bias in the direction of peeling toner from the
developing roll to the magnetic roll with switching after formation
of images (JP1995-72733) and JP1995-92804 or at a definite interval
(JP1994-67546) and by applying a direct current bias (JP1995-72733)
in the direction of transferring toner on the magnetic roll to the
developing roll with switching at the successive image formation,
whereby formation of images are prepared to solve the above
mentioned problems.
[0018] Japanese laid-open patent publication JP1995-72733, U.S.
Pat. No. 5,420,375, disclose a hybrid developing apparatus having a
magnetic roll forming magnetic brushes by two-components developer
material, a donor roll (developing roll) holding a toner thin layer
supplied by the magnetic roll and electrodes disposed between the
donor roll and a latent image bearing body wherein a bias
consisting of direct current and alternate current is applied to
the electrodes and to the magnetic roll is applied direct current
bias capable of changing polarity with a switch whereby such
problems as decreasing of image consistency owing to selective
developing or developing defect, image degradation, developing
ghost, toner scattering and sleeve adhesion owing to leaving for a
long time while holding toner on a developing roll are solved.
[0019] That is to say, in the developing apparatus disclosed in
JP1995-72733, a toner thin layer is formed on a magnetic roll with
a potential difference between the magnetic roll and a developing
roll and a latent image on the latent image bearing body is
developed by forming toner cloud near the electrodes with an
alternate current bias superposed on direct current applied between
the developing roll and the electrodes while the toner on the
developing roll is recovered by applying a direct current bias in
the direction of peeling toner from the developing roll to the
magnetic roll with switching after formation of images and by
applying a direct current bias in the direction of transferring
toner on the magnetic roll to the developing roll with switching at
the successive image formation, whereby formation of images are
prepared to solve the above mentioned problems.
[0020] In JP2000-250294, it is argued that a method in which
electrodes are provided between a developing roll and a latent
image bearing body disclosed in JP1995-72733 has such draw back as
an uneven phenomenon owing to vibration of wires tensed with
electrical bias and generation of scars on the developing roll by
dust attached instantaneously on the electrodes. A hybrid
developing apparatus having electrodes buried in a developing roll
is introduced as a prior art. Even in the apparatus using a
developing roll with electrodes buried in the roll, carrier
attached on the developing roll sticks to images and toner is not
effectively supplied to the latent image bearing body because
electrodes buried in the roll have a certain distance between them.
Consequently, the apparatus generates a phenomenon of empty images
when images of high image ratio are continuously printed or a
phenomenon of low quality image or low image consistency caused by
selective development on account of alternate current bias applied
to the magnetic roll and the developing roll so that the patent
publication disclosed a hybrid developing apparatus preventing
these phenomenon.
[0021] Namely, in the patent publication JP2000-250294, a
dielectric layer into which electrodes are buried with a small
distance is provided on a electroconductive sleeve to constitute a
developing roll whereby improving the effectiveness of supplying
toner to a latent image bearing body, preventing short-circuiting
between electrodes and protecting the surface. In addition, an
electrode-relaxation-allowing layer having a dielectric constant
with which cumulated electric charges can be diffused and capable
of fringe charge penetrating through the coating for a shorter time
than several seconds is provided. A brush electrode supplying a
bias voltage consisting of alternate current voltage and dielectric
current voltage to the electrodes buried in the developing roll is
provided between the developing roll and the latent image bearing
body and another brush electrode supplying another bias voltage
consisting of alternate current voltage and dielectric current
voltage to the electrodes buried in the developing roll is provided
between the developing roll and magnetic roll, so that, a latent
image on the latent image bearing body is developed with a cloud of
toner between the latent image bearing body and the developing
roll, and toner is reciprocated between the developing roll and the
magnetic roll. The alternate current voltage source is common to
the both biases and its wave is a square wave, a duty ratio of
which is set in such a manner that a time for transferring toner
from the magnetic roll to developing roll is shorter than a time
for recovering toner from developing roll to the magnetic roll so
as to prevent selective transfer of toner and carrier sticking to
the developing roll utilizing the difference of inertia between
toner and carrier.
[0022] The powder cloud developing method disclosed in the Japanese
laid open patent publication, No. JP1992-113474 is not commonly
practiced because weirs of auxiliary electrodes are apt to become
dirty and image degradation generates owing to vibration. Further,
the apparatuses disclosed in JP1999-231652, JP2000-81788 and
JP1995-128983 have following drawbacks; namely, one needs a
scratching device for toner and a recovery roll; another has a
cause of degradation of toner durability owing to increasing toner
stress by applying a special bias for recovery; yet another needs a
long time for forming the layer on the developing roll at the time
of developing reducing a developing speed. Further, after
processing for a long time, charging property of toner varies
greatly on the developing roll owing to degradation of durability
of carrier leading to broadening the charging distribution of
replenishing toner or recovered toner, which results in toner
scattering or image fogging caused by poor toner charge. In
addition, as the art needs a troublesome exchange of carrier, it
does not have turned to a practical use.
[0023] In a tandem type developing apparatus disclosed in
JP1988-249164, the action of developing apparatus except for an
image forming part processing a transfer process needs to be
stopped or the apparatus needs an apparatus or an control device
for changing a high voltage applied between the developing roll and
the magnetic roll with a high frequency resulting in cost up, while
the apparatus has a structure of disposing donor roll, a magnetic
roll and a stirrer member in a horizontal way so that downsizing of
the apparatus is difficult. In a developing apparatus disclosed in
JP1998-31366, a non-contact, two-component developer material
method is carried out, while a plurality sets of developing
magnetic poles on the magnetic roll and a trimming magnetic pole
controlling a toner replenishing amount or a complex structure
capable of fixing rotatably at a plurality of positions according
to a replenishing amount of toner need to be provided inevitably
leading to a complicated control method and a high cost developing
apparatus.
[0024] An apparatus disclosed in JP1994-67546, JP1995-72733 or
JP1995-92804 is operated by the powder cloud method in which the
wires of the auxiliary electrodes are likely to get dirty and image
degradation caused by vibration occurs. The apparatus disclosed in
JP2000-250294 needs to bury electrodes into the developing roll and
has a complicated and costly structure as necessary for a brush
electrode for supplying the electrodes a superposed bias of AC and
DC. The electrodes disposed intermittently in the circumferential
direction while toner can not be controlled when the brush
electrode becomes unable to contact the electrodes on the
developing roll owing to sticking of toner caused by vibration or
getting dirty for some reasons.
[0025] Further, as mentioned above, when high consistency images
are printed successively, fine particles of toner and contamination
of toner constituents stick to the developing roll that causes
toner filming leading to unevenness of images such as generation of
heterogeneous image consistency. These are problems when downsizing
the developing apparatus.
SUMMARY OF THE INVENTION
[0026] The present invention was done for solving the
above-mentioned problems. An object of the present invention is to
provide a hybrid developing method wherein developing ghost and
selective developing are avoided without complicating a developing
apparatus and surly charged toner is supplied to a developing roll
while unevenness of images is not generated to obtain stable images
for a long time even when continuous printing.
[0027] Another object of the present invention is to provide an
image forming method in a hybrid developing apparatus appropriate
for a color tandem type image forming apparatus wherein stable
quality images are obtained for a long time.
[0028] In order to attain these objects, according to the present
invention, a method for developing in a hybrid developing apparatus
having a magnetic roll for transferring two-component developer
material, which electrostatically charges developer material
consisting of carrier and toner magnetically held thereon, and a
developing roll, which forms a toner alone thin layer on its
surface by transferring the toner utilizing a bias for transferring
and magnetic brushes of the magnetic roll wherein latent images on
a latent image bearing body are developed with the toner by
applying developing bias to nearest approached regions (developing
regions) between the developing roll and the latent image bearing
body, is characterized in that a data amount of each image, which
an image forming apparatus prints and a number of paper sheets
passed are detected and that an interval of the action of
recovering and exchanging a toner layer on the developing roll to
the magnetic roll after printing (between paper sheets) is variably
set in accordance with a printing coefficient determined by the
data amount and the number of paper sheets passed.
[0029] Thus, by variably setting an interval of the action of
recovering and exchanging a toner layer on the developing roll to
the magnetic roll after printing (between paper sheets) in
accordance with a printing coefficient determined by the data
amount and the number of paper sheets passed, toner filming caused
by sticking fine particles or contamination of toner constituents
is prevented even when images having consistency over 50% are
successively printed. The unevenness of images such as generation
of heterogeneous image consistency or unevenness of the toner layer
caused by the toner filming can be avoided. When images with high
consistency are successively printed, poor toner charge caused by
selective developing of toner or deficiency of toner amount on the
developing roll occurs so as to diminish toner performance
regarding developing. According to the present invention, an
appropriate amount of toner can constantly be supplied onto the
developing roll so that generation of heterogeneous image
consistency can be avoided while keeping enough printing rate.
Therefore, excellent images can be obtained for a long time,
preventing from generation of ghost, from staining of the
developing roll or from poor performance for following solid
images.
[0030] In this case, a rotational rate of the developing roll is
1.1 times or more that of the latent image bearing body so that
enough toner is supplied to a latent image on the latent image
bearing body without broadening the distance between paper sheets,
printing rapidly; and a rotational rate of the developing roll is
1-2 times that of the magnetic roll so that toner exchanging on the
developing roll is enhanced, avoiding stress to toner caused by
insufficient recovery of toner, vibration or heat generation.
[0031] Further, changing rates of the volume average particle size
of the toner on the developing roll is controlled within 15% by
recovering and exchanging a toner layer on the developing roll to
the magnetic roll so that generation of mottle in image consistency
is prevented while a sufficient printing rate is maintained, and
ghost, staining of the developing roll and faulty performance for
following solid images are prevented, providing excellent images
for a long time.
[0032] Even if a printing coefficient varies, uniform images are
obtained by changing a toner amount on the developing roll within
the range from 0.5 to 1.7 mg/cm.sup.2. For example, in ordinary
printing, toner scattering is diminished by setting a toner amount
on the developing roll as comparatively small; in high consistency
printing, an appropriate toner layer at the beginning of developing
can be formed by supplying an enough amount of toner to the
developing roll so that uniform images are always obtained.
[0033] Further, selective developing is avoided by controlling an
amount of toner charge on the developing roll at the beginning of
developing within the range from 5 to 20 .mu.C/g, and toner
scattering and diminishing of toner performance regarding
developing are avoided by controlling a toner layer thickness to an
optimum value, leading to obtaining excellent images for a long
time.
[0034] Further, according to the present invention, the surface of
the developing roll consists of an approximately uniform electro
conductive sleeve having a conductivity of 10.sup.6
.OMEGA..multidot.cm.sup.3 or less to which a bias is applied. The
applied bias is acted between the developing roll and the latent
image bearing body and between the developing roll and the magnetic
roll so that a toner layer on the developing roll is recovered and
exchanged to the magnetic roll after printing (between paper
sheets). Thus, the bias applied to the developing roll acts
favorably to the gap between the rotating developing roll and the
latent image bearing body and the magnetic roll so that image
consistency does not decrease, leading to obtaining excellent
images for a long time.
[0035] When a plurality of the aforementioned hybrid developing
apparatuses for all different colors of toner are disposed along
the moving direction of a transfer medium of images for applying to
a color tandem type image forming apparatus, it is preferable that
after toner layers on the developing rolls of the all developing
apparatuses are peeled off to transfer toward the magnetic rolls
after developing, a thickness of the toner layer of the each
developing apparatus is variably controlled at the beginning of
developing in accordance with the image consistency of the
developing apparatus having a highest image consistency among image
data of toner colors. Image consistency herein is a value corrected
by a filter corresponding to each color with a Macbeth
densitometry.
[0036] According to the present invention, a toner layer thickness
in accordance with image consistency can always be obtained by
calculating an image data amount of each toner color printed after
toner layers on the developing rolls of the all developing
apparatuses are peeled off to transfer toward the magnetic rolls
after developing and by controlling variably a thickness of the
toner layer of the each developing apparatus at the beginning of
developing in accordance with the image consistency of the
developing apparatus. Thus, image unevenness such as heterogeneous
image consistency or non-uniform toner layer, which generates when
high consistent images having a consistency of 50% or greater are
successively printed, can be prevented. When images with high
consistency are successively printed, poor toner charge caused by
selective developing of toner or deficiency of toner amount on the
developing roll occurs so as to diminish toner performance
regarding developing. Even in this case, an appropriate amount of
toner can constantly be supplied onto the developing roll so that
generation of heterogeneous image consistency can be avoided while
keeping enough printing rate. Therefore, an image forming method in
a color tandem type image forming apparatus in which excellent
images can be obtained for a long time, preventing from generation
of ghost, from staining of the developing roll or from poor
performance for following solid images can be provided.
[0037] According to an embodiment of a toner layer thickness
controlled variably, a toner layer thickness at the beginning of
developing is {fraction (1/10)}-{fraction (1/4)} of the gap between
a developing roll and a latent image bearing body; a toner amount
of the toner layer on the developing roll controlled variably at
the beginning of developing is controlled to the range from 0.5 to
1.5 mg/cm.sup.2; and an amount of toner charge on the developing
roll at the beginning of developing is controlled to the range from
5 to 20 .mu./g. Thus, even if a data amount of images varies, even
images are obtained. For example, toner scattering is diminished by
setting a toner amount on the developing roll comparatively small
in case of an ordinary printing and an appropriate toner layer is
formed on the developing roll at the beginning of developing by
supplying a sufficient amount of toner to the developing roll in
case of a high consistent printing so as to always obtain even
images. With the above method, a selective developing is prevented
and toner scattering and decrease of developing performance are
prevented by controlling a toner layer thickness to an optimum
value so that a color tandem type image forming apparatus giving
excellent images for a long time is provided.
[0038] Sufficient toner is supplied to a latent image on the latent
image bearing body without enlarging a distance between paper
sheets by rotating the developing roll 1.5 to 3 times as rapidly as
the latent image bearing body so that a rapid printing can be
performed.
[0039] Further according to the present invention, the developing
bias consisting of a direct current bias and an alternate current
bias superposed thereon is applied onto an electroconductive sleeve
on the surface of the developing roll, the sleeve having a volume
resistivity of 10.sup.6 .OMEGA..multidot.cm.sup.3 or smaller, so as
to recover the toner layer on the developing roll to the magnetic
roll after developing whereby the bias of a direct current bias and
an alternate current bias superposed thereon acts favorably to the
gap between the rotating developing roll and latent image bearing
body, leading to obtaining a stable image for a long time without
decreasing an image consistency.
BRIEF DESCRIPTION OF THE DRAWINGS
[0040] FIG. 1 is a schematic drawing illustrating an embodiment of
a color tandem type image forming apparatus carrying out an image
forming method according to the present invention.
[0041] FIG. 2 is a schematic drawing illustrating a hybrid
developing apparatus carrying out an image forming method according
to the present invention.
[0042] FIG. 3 is a schematic drawing illustrating generation of
ghost.
[0043] FIG. 4 is a table showing a relationship between a volume
resistivity of a sleeve on a developing roll and an image
consistency after printing 10,000 sheets (initial 1.43).
[0044] FIG. 5 is a graph showing a relationship between a
rotational rate of a developing roll and a weight of a toner
layer.
[0045] FIG. 6 is a schematic drawing illustrating a state of a
thickness of a toner thin layer on the developing roll, ghost,
scattering and performance for following solid images.
[0046] FIG. 7 is a table showing a state of generation of ghost and
staining of the developing roll comparing with the case toner is
exchanged and the case toner is not exchanged after printing
continuously 10,000 sheets of printing data of 5% of image
data.
[0047] FIG. 8 is a table showing a state of consistency and
performance for following solid images comparing with the case
toner is exchanged and the case toner is not exchanged after
printing continuously 100 sheets of solid printing data of 100% of
image data.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
[0048] The invention will now be described below in detail by way
of example with reference to the accompanying drawings. It should
be understood, however, that the description herein of specific
embodiments such as to the dimensions, the kinds of material, the
configurations and the relative disposals of the elemental parts
and the like is not intended to limit the invention to the
particular forms disclosed but the intention is to disclose for the
sake of example unless otherwise specifically described.
[0049] In FIG. 1, 1 is a tandem type image forming apparatus having
main units 1A-1D forming images of cyan, magenta, yellow and black
and disposed along the transporting direction of a transporting
belt for recording media. Each main unit 1A-1D includes a
developing apparatus 2 having a developing container 6, a latent
image bearing body (photosensitive body) 3, an exposing unit 4, a
charging device 8 for charging the latent image bearing body 3 and
a transferring device 9 for transferring toner images on the latent
image bearing body 3 by means of a transferring bias, mounted
therein.
[0050] 7 is a cassette for supplying paper disposed under the main
unit 1A-1D, having a structure layered vertically in three and
leading a recording paper to a resist roller 7b through a
paper-supplying roller 7b. By means of rotation of a resist roller
7b, a recording medium is transported on a route for transporting
recording media, while color toner images are transferred by main
units 1A-1D forming images of cyan, magenta, yellow and black. 10
is a fixing device fixing the color toner images.
[0051] In a color tandem type image forming device, it is important
to design the devices disposed around the latent image bearing body
(photosensitive body) 3 compact, the devices such as a charging
device 8, an exposing unit 4, a developing apparatus 2,
transferring device 9 and cleaning device. According to the present
invention, a developing apparatus 2 is disposed adjacent to the
latent image bearing body 3 and in a vertical direction.
[0052] In FIG. 2, 21 is a magnetic roll as a transporter for
developer material generating magnetic brushes 30 of two-component
developer material by a magnet disposed therein and supplying toner
25 to a developing roll 22; 22 is a developing roll holding a toner
thin layer, which develops latent images on the latent image
bearing body 3; 24 is carrier; 25 is toner; 26 is a toner thin
layer on the developing roll 22; 27 is a developing bias for
developing by causing toner 25 of the toner thin layer 26 formed on
the developing roll to jump to the latent image bearing body, which
is composed of a DC bias 27a (V.sub.dc1) and an AC bias 27b; 28 is
a DC bias (V.sub.dc2) for transporting toner 25 from the magnetic
roll to the developing roll 22; and 29 is a restricting blade for
restricting the thickness of magnetic brushes 30 on the magnetic
roll 21.
[0053] As a material for latent image bearing body (photosensitive
body) 3, an amorphous silicon (a-Si) photosensitive material, an
organic photosensitive material (OPC) or the like may be used.
Since an organic photosensitive material taking a positive charge
(positive OPC) generate little ozone and the like, being stable in
charge. Particularly, an organic photosensitive material taking a
positive charge having a single layer structure is appropriate for
a long life system because it does not varies in photosensitive
characteristics even in case the film thickness is varied after
long use so that image quality is stable. When an organic
photosensitive material taking a positive charge is used in a long
life system, its film thickness is preferably 20 .mu.m to 40 .mu.m.
In case a film thickness of 20 .mu.m or under is used, black spots
are generated owing to dielectric breakdown when the thickness is
decreased to approximately 10 .mu.m. In case a film thickness of 40
.mu.m or greater is used, a sensitivity is decreased causing
decline of an image consistency.
[0054] As an exposing unit 4, a semiconductor laser or a LED can be
used. When an organic photosensitive material taking a positive
charge is used, a wavelength of 770 nm or vicinity is effective.
When amorphous silicone photosensitive material is used, a
wavelength of 685 nm or vicinity is effective. A case of using a
LED as a light source of the exposing unit 4 and using an organic
photosensitive material taking a positive charge as the latent
image bearing body 3 is explained hereinafter according to the
present invention.
[0055] A most surface of the developing roll comprises a sleeve
consisting of uniformly conductive aluminum having a volume
resistivity of 10.sup.6 .OMEGA..multidot.cm.sup.3 or under, SUS, or
electroconductive film and the like. To the shaft part thereof is
applied a DC bias (V.sub.dc1) 27a and AC bias 27b so as to act the
bias of the DC bias and the AC bias superposed thereon to the gap
between the rotating developing roll 22 and the latent image
bearing body 3 and to the gap between the rotating developing roll
22 and the magnetic roll 21. The AC component which the AC bias
supplies comprises square waves having a duty ratio of 50% or less.
According to the present invention, the DC bias (V.sub.dc1) 27a is
set as 100 V; the V.sub.pp, the frequency and the duty ratio of the
AC bias 27b are set as 1.5 kV, 3.0 kHz and 30% respectively as an
example. As stated above, the DC bias (V.sub.dc1) 27a and the AC
bias 27b are applied directly to the developing roll 22 and the
most surface of the developing roll consists of an
electro-conductive material having a volume resistivity of 10.sup.6
.andgate..multidot.cm.sup.3 so that sharp bias components can be
applied between the developing roll 22 and the latent image bearing
body 3, and between the developing roll and the magnetic roll 21
whereby enhancing reactivity of toner layer formation at the
beginning of developing. A specific bias can be applied for
adjusting formation of the toner layer at the time of formation of
the toner layer till the beginning of developing depending on image
data.
[0056] The magnetic roll 21 holds two-component developer material
consisting of carrier 24 and toner 25 as a transporter for
developer material, generating magnetic brushes 30 whose thickness
is restricted by the restricting blade 29 and supplying the toner
25 to the developing roll 22. A gap between the restricting blade
and the magnetic roll is preferably 0.3 to 1.5 mm. The toner 25 is
supplied to the developing roll by a potential difference between
the DC bias (V.sub.dc2) 28 applied to the magnetic roll and DC bias
(V.sub.dc1) 27a applied to the developing roll and by the AC bias
27b. The voltage of the DC bias (V.sub.dc2) 28 is 350 V as a
example of the present invention and in order that the toner layer
on the developing roll 22 may be exchanged after developing, the DC
bias (V.sub.dc2) 28 is converted as the AC bias 27b is applied so
as to recover the toner thin layer 26 on the developing roll 22 to
the magnetic brushes 30. A value changed of the DC bias (Vdc2) 28
is kept greater than a value which generates a potential in the
direction for always transferring the toner of the magnetic brushes
30 to the developing roll 22, that is, a DC bias voltage to the
magnetic roll is kept greater than that of the developing roll and
if the DC bias (V.sub.dc1) is 100 V, the value is greater than 100V
(as for this example, the value between 100 V and 350 V). Regarding
the variation of the DC bias, though the DC bias (V.sub.dc2) 28 is
converted in the above example, both a DC bias (V.sub.dc2) 28 and a
DC bias (V.sub.dc1) 27a can be converted if a DC bias voltage of
the magnetic roll 21 is kept greater than a DC bias voltage of the
developing roll.
[0057] Conductivity of the sleeve on the developing roll 22 is
related to maintaining property of printing consistency. The
variation of image consistency (initial 1.43) after printing 10,000
sheets is shown in FIG. 4. A DC bias (V.sub.dc1) 27a applied t the
developing roll 22 is 100 V; A V.sub.pp, a frequency and a duty
ratio of the AC bias are 1.5 kV, 3.0 kHz and 30% respectively; and
a DC bias (V.sub.dc2) 28 is 350V. As a result: if a volume
resistivity of the sleeve material is 10.sup.7
.OMEGA..multidot.cm.sup.3 or greater, image consistency after
printing 10,000 sheets decreases extremely; and if it is 10.sup.6
.OMEGA..multidot.cm.sup.3 or less, there is almost no problem with
regard to image consistency.
[0058] A gap between the latent image bearing body 3 and the
developing roll 22 is approximately 250 .mu.m as an example and no
wire electrode is used between them. A gap between the latent image
bearing body 3 and the developing roll 22 is usually 150 .mu.m to
400 .mu.m , preferably 200 .mu.m to 300 .mu.m. If the gap is less
than 150 .mu.m, it causes fogging. If it is grater than 400 .mu.m,
it becomes difficult to cause the toner 25 to jump to the latent
image bearing body 3 so that enough image consistency cannot be
obtained and causes selective developing. A gap between the
magnetic roll 21 and the developing roll 22 is approximately 0.3
.mu.m to 1.5 mm .mu.m.
[0059] Developer material consists of toner 25 and carrier 24. It
is important that the toner 25 is restricted with regard to its
particle size distribution. Generally, a particle size distribution
of toner is measured by a Coulter counter and a breadth of particle
size distribution of a toner is expressed as a ratio of a volume
average particle size and a number average particle size. In order
to prevent selective developing, it is important to make the ratio
small. The broader the distribution becomes, the more toner having
a comparatively small particle size is deposit on the developing
roll so that performance for developing decreases. According to the
present invention, an amount of toner charge is controlled within
the range of 5 .mu.C/g to 20 .mu.C/g to prevent selective
developing. The amount of toner charge is related also to a layer
thickness of the toner thin layer 26 formed on the developing roll
22 so that the thickness of the toner layer becomes thick when the
amount of toner charge is as low as 10 .mu.C/g or less,
particularly 5 .mu.C/g or less, leading to increasing scattering of
toner. While, when the amount of toner charge is 20 .mu.C/g or
greater, the toner layer thickness becomes thin so that toner
performance regarding developing decreases owing to high charge.
The amount of toner charge is measured by sucking the toner thin
layer 26 on the developing roll with a QM meter of Trek Co.. In the
present invention, a case of using positively charged toner is
explained as an example. However, negatively charged toner may be
also used by reversing the relation to the bias.
[0060] As for carrier 24, magnetite carrier, ferrite of Mn series,
ferrite of Mn-Mg series can be used and surface treated material
may also be used unless an appropriate resistivity does not
increase. According to the present invention, silicone resin
treated ferrite carrier having a volume resistivity of 10.sup.8
.OMEGA..multidot.cm.sup.3, a saturation magnetization of 70 emu/g
and an average particle size of 35 .mu.m is used as an example.
When the average particle size exceeds 50 .mu.m, the stress of
carrier increases while the toner consistency can not be raised so
that the toner supply to the developing roll diminishes. When the
average particle size of carrier is less than 50 .mu.m, enough
charge is given so that stable developing can be carried out even
when the toner concentration in the developer material ranges from
5 to 20%.
[0061] The mixing ratio of toner 25 and carrier 24 is from 5 to 20
weight % of toner per total amount of toner 25 and carrier 24,
preferably from 5 to 15 weight %. When the mixing ratio of toner 25
is less than 5%, the amount of toner charge becomes high so that
enough image consistency can not be obtained, while, when it
exceeds 20%, toner can not get enough charge so that toner scatters
from a developing device contaminating the inner part of an image
forming apparatus or generating toner fogging on the images.
[0062] The thickness of a toner 25 thin layer 26 ranges from 10 to
100 .mu.m, preferably from 30 to 70 .mu.m. The thickness
corresponds to 5 to 10 layers of the toner 25 and to {fraction
(1/10)} to {fraction (1/4)} of the gap between the developing roll
and the latent image bearing body (150 to 400 .mu.m, preferably 200
to 300 .mu.m). Thus, according to the present invention, the
thickness of the toner 25 layer formed on the electro-conductive
sleeve on the developing roll is 50 .mu.m or less and the toner
amount is controlled within the range of from 0.5 to 1.7
mg/cm.sup.2, preferably 0.5 to 1.5 mg/cm.sup.2 whereby forming
clear images on the latent image bearing body 3 and at the same
time easing the exchange of toner on the developing roll 22, which
result in controlling developing ghosts.
[0063] Further according to the present invention, in a series of
image forming processes, clear images are formed on the latent
image bearing body 3 and high consistent data can be continuously
printed by controlling the thickness of the toner 25 layer formed
on the sleeve of the developing roll 22 to 50 .mu.m or less and the
amount of the toner to within the range of from 0.5 to 1.7
mg/cm.sup.2, preferably from 0.5 to 1.5 mg/cm.sup.2.
[0064] The thickness of the toner thin layer 26 is determined by
the difference between the DC bias (V.sub.dc2) 28 and the DC bias
(V.sub.dc1) 27a and the amount of toner charge. In case the amount
of toner charge is as low as 10 .mu.C/g or less, particularly 5
.mu.C/g or less, the toner layer becomes thin, rising the charge so
that toner performance regarding developing diminishes. As
described above, when the DC bias (V.sub.dc1) 27a is set as 100 V
and the DC bias (V.sub.dc2) 28 as 350 V, the toner layer having a
thickness of 40 .mu.m is obtained. The toner amount per unit area
in this condition is about 1.0 mg/cm.sup.2. When the toner thin
layer 26 is as thin as 0.5 mg/cm.sup.2 or less, the following
property of consistency is declined in case of successive printing
of high consistent images, resulting in liability of generation of
uneven images. When the toner layer is as thick as greater than 1.5
mg/cm.sup.2, developing ghost as shown in FIG. 3 and toner
scattering become noticeable. That is, as described above, in the
FIG. 3, 35 is a rectangular, black and solid image. 36 and 37 are
half tone images which succeed it and are broader than the former
solid image 35. When a toner consumption region and a toner
non-consumption region are generated on the developing roll and the
half tone images 36, 37 are printed successively after printing the
solid image 35, a ghost as 38 shown in FIG. 3(b) is generated.
[0065] In order to exchange the toner thin layer 26 on the
developing roll, the toner thin layer 26 on the developing roll 22
is recovered to the magnetic brushes 30 by varying the DC bias
(V.sub.dc2) 28 in the state of applying the AC bias 27b. The
exchange of toner on the developing roll 22 is enhanced by rotating
the magnetic roll 21 one to two times as fast as the developing
roll. It is preferable that the magnetic roll 21 rotates in the
same direction as the developing roll 22; for example, when the
developing roll 22 rotates counterclockwise, the magnetic roll
rotates also counterclockwise. For supplying a latent image on the
latent image bearing body 3 enough toner, toner is exchanged
rapidly when the circumference rotating rate of the developing roll
22 is set 1.5 to 3 times as fast as that of the latent image
bearing body 3. When the rate ratio is 1.5 times or less, time when
developing begins is late so that an enough sheets distance is
required, resulting in failure in rapid printing. When the rate
ratio is 3 times or greater, vibration or heat is generated because
of the increased rotation of the developing roll 22.
[0066] Toner performance for developing decreases sometimes when
high consistent printings, for example, over 50% are successively
performed with a compact type developing apparatus. One possible
reason is selective developing of toner; another is deficiency of
toner on the developing roll. As a measure for selective
developing, it is proposed that toner is made return from
developing roll 22 to the magnetic roll 21 by reverse electric
field after developing. However the possibility of generating
unevenness of image consistency becomes large because of deficiency
of toner on the developing roll, which is the other possible
reason.
[0067] Hence, to stabilize the image consistency, when printing
data are transmitted, an image data amount of the printing data is
calculated by a dot counter having an unshown control device of the
image forming apparatus and the layer thickness of the toner thin
layer 26 on the developing roll 22 is controlled in accordance with
the image data amount. FIG. 5 is a graph showing a relationship
between a number of rounds of the developing roll 22 and a weight
of the toner thin layer 26 formed on the developing roll 22. In
FIG. 5: a number of rounds of the developing roll 22 is plotted in
horizontal axis and a weight of the toner layer 26 on the
developing roll 22 in vertical axis; and the solid line denotes
toner weights appropriate for image data of ordinary printing
coefficient, and the dotted line denotes toner weights appropriate
for image data of high printing coefficient. As for the DC bias
(V.sub.dc1) 27a applied to the developing roll 22 and the DC bias
(V.sub.dc2) applied to the magnetic roll 21, the DC bias
(V.sub.dc1) 27a is set as 100 V, and the DC bias (V.sub.dc2) 28 is
set as 350 V in case of solid line of image data of ordinary
printing coefficient and 400 V in case of doted line of image data
of high printing coefficient.
[0068] As shown in the graph, while a minimum toner weight needed
on the developing roll 22 is about 0.7 to 0.8 g/cm.sup.2, in case
of the solid line corresponding to the ordinary printing
coefficient where the DC bias (V.sub.dc1) 27a is 100 V and the DC
bias (V.sub.dc2) 28 is 350 V, the toner weight becomes 0.4 to 0.5
mg/cm.sup.2 after one round of the developing roll 22 and it
surpasses the minimum necessary amount of 0.7 to 0.8
mg/cm.sup.2after its two rounds. In case of the doted line
corresponding to the high printing coefficient, the toner weight
becomes 0.6 to 0.7 mg/cm.sup.2 after one round of the developing
roll 22 and 0.9 to 1.0 mg/cm.sup.2 after its two round so that it
surpasses also the minimum necessary amount of 0.7 to 0.8
mg/cm.sup.2. Thus, the toner amount on the developing roll can be
varied by controlling the voltage value of DC bias (V.sub.dc2)
28.
[0069] That is, as stated above, the rotational rate of the
magnetic roll 21 is set as one to two times as fast as that of the
developing roll 22, the rotational direction of the magnetic roll
is set as the same direction as that of the developing roll 22, and
the voltage value of the DC bias (V.sub.dc2) 28 of the developing
roll 22 is controlled, so that the layer thickness of the toner
thin layer 26 on the developing roll is controlled to {fraction
(1/10)} to {fraction (1/4 )} of the gap between the developing roll
and the latent image bearing body 3 (150 to 400 .mu.m, preferably
200 to 300 .mu.m) in accordance with the data amount of printing
data. Thus, for successive printings, the printing coefficient of
printing data is calculated by a dot counter provided with a
control device of the image forming apparatus. The DC bias (Vdc2)
28 is varied by the printing coefficient so as to obtain an even
image. The above-mentioned process is important for preventing
toner scattering from the developing roll 22. Toner scattering is
decreased by setting toner amount on the developing roll as
comparatively little at the time of ordinary printing. The toner
amount on the developing roll is adjusted only when it is
necessary, such as at the time of high consistent printing so that
an optimum toner layer can be formed at the beginning of
developing.
[0070] In FIG. 6, ghost, scattering and performance for following
solid images are shown by varying the thickness (mg/cm.sup.2) of
the toner thin layer on the developing roll 22. .largecircle.
denotes good performance, .DELTA. denotes the case where the
defects are observed a little and .times. denotes bad performance.
As shown in FIG. 6: when the toner layer is too thin as 0.4
mg/cm.sup.2, performance for following solid images is declined;
when the toner layer is too thick as over 1.7 mg/cm.sup.2,
developing ghost and toner scattering are inclined to be apparent.
Thus, the thickness (mg/cm.sup.2) of the toner thin layer 26 is
preferably approximately 0.5 to 1.5 (mg/cm.sup.2).
[0071] In a tandem type image forming apparatus having a hybrid
developing apparatus thus constructed according to the present
invention, two components developer material consisting of toner 25
corresponding to an individual color such as yellow, cyan, magenta
or black and carrier 24 is supplied to the developing apparatus 2
from the developing container 6, forming the magnetic brushes 30 on
the magnetic roll 21 as shown in FIG. 2 and charging the toner 25
by stir. The magnetic brushes 30 on the magnetic roll 21 is
restricted with its layer thickness by the restricting blade 29 and
the thin layer of the only toner 25 is formed on the developing
roll 22 by the potential difference between the DC bias (V.sub.dc2)
28 applied to the magnetic roll 21 and the DC bias (V.sub.dc1) 27a,
and AC bias 27b.
[0072] When the signal for beginning printing is transmitted from
an unshown control circuit, the latent image bearing body 3
comprising organic photosensitive material taking a positive charge
(positive OPC) is charged at first by the charging device 8, for
example to 400 V and after that, the latent image bearing body 3 is
exposed by LED which an exposing unit comprises and having a wave
length of 770 nm to a potential of 70 V after exposing to form a
latent image. The latent image is developed to form a toner image
with toner caused to jump to the latent image bearing body 3 from
the toner thin layer 26 on the developing roll 22 by the DC bias
(V.sub.dc1) 27a and AC bias 27b.
[0073] By the time a toner image is formed on the latent image
bearing body 3, a recording medium is taken from a cassette for
supplying paper and transported onto a transporting belt 5. A toner
image is transferred by applying a transfer bias by a transfer
device 9 disposed at the transfer position of individual color. The
toner image of each color is sequentially transferred to the
recording medium, which reaches a fixing device 10, is fixed, and
discharged.
[0074] The AC bias 27b applied to the developing roll is a square
wave having a duty ratio of 50% or less. The period for
transporting toner 25 from the magnetic roll 21 to the developing
roll is shorter than the period for recovering toner 25 from the
developing roll 22 to the magnetic roll 21. Because the DC bias
(V.sub.dc1) 27a and the AC bias 27b is applied directly to the
developing roll and the most surface of the developing roll 22 is
made of electro-conductive material having a volume resistivity of
10.sup.6 .OMEGA..multidot.cm.sup.3 or less, AC component of the
bias for recovering the toner thin layer 26 on the developing roll
to the magnetic roll 21 has a sharp peak so as to recover the toner
25 effectively. Therefore, the effectiveness of recovering toner
from developing roll 22 to the magnetic roll 21, which is a
transporter for developer material, is enhanced so that staining of
the developer roll with toner is prevented and recovery of toner
from the latent image bearing body 3 toward the developing roll is
effectively performed, which result in providing a developing
apparatus capable of obtaining quality images having long stability
and preventing generation of fogging.
[0075] Further, according to the present invention, when printing
data are transmitted, a printing coefficient of the image data of
the individual toner color used in the color tandem type image
forming apparatus is calculated by a dot counter having an unshown
control device of the image forming apparatus and the toner thin
layer 26 is peeled off from the developing roll 22 for refreshing
while the layer thickness of the toner is variably controlled at
the beginning of developing in accordance with the image
consistency of a developing apparatus where the image consistency
is highest. The refreshing is carried out in such a manner that the
toner thin layer 26 on the developing roll 22 is recovered to the
magnetic brushes 30 by varying the DC bias (V.sub.dc2) 28 in the
state of applying the AC bias 27b and the thickness of the toner is
varied by getting back to a voltage in accordance with the image
consistency at the beginning of developing. Thus, the layer
thickness of the toner thin layer 26 on the developing roll is
controlled to {fraction (1/10)} to {fraction (1/4)} of the gap
between the developing roll and the latent image bearing body 3
(150 to 400 .mu.m, preferably 200 to 300 .mu.m) so as to prevent
generation of ghost, staining of developing roll and poor
performance for following solid images, whereby a method for
developing in a compact color tandem type image forming apparatus
capable of giving quality images for a long time can be
provided.
[0076] A voltage value of the DC bias (V.sub.dc1) 27a, the AC bias
27b or the DC bias 28, V.sub.pp or the frequency that has been
explained above is an example and is obviously variable according
to the situation.
[0077] Hence, according to the present embodiment, after action of
peeling the toner layer from the developing roll toward the
magnetic roll is carried out at the end of developing, an amount of
the image data of individual toner color which is printed is
calculated and the layer thickness of the toner is variably
controlled at the beginning of developing in accordance with the
image consistency of a developing apparatus where the image
consistency is highest so that a toner layer corresponding to an
image consistency is always obtained. Therefore, unevenness of
image such as mottle in image consistency or unevenness of the
toner layer which occurs when high consistent images, for example,
having an image consistency of 50% or greater, are successively
printed can be avoided. When images with high consistency are
successively printed, poor toner charge caused by selective
developing of toner or deficiency of toner amount on the developing
roll occurs so as to diminish toner performance regarding
developing. Even in this case, an appropriate amount of toner can
constantly be supplied onto the developing roll so that generation
of ghost, staining of the developing roll or poor performance for
following solid images is prevented, whereby excellent images are
obtained for a long time.
[0078] Further, according to the present invention, a toner layer
thickness at the beginning of developing is {fraction
(1/10)}-{fraction (1/4)} of the gap between a developing roll and a
latent image bearing body; a toner amount of the toner layer is
controlled to the range from 0.5 to 1.5 mg/cm.sup.2; and an amount
of toner charge is controlled to the range from 5 to 20 .mu./g.
Thus, even if a data amount of images varies, even images are
obtained. For example, toner scattering is diminished by setting a
toner amount on the developing roll relatively small in case of an
ordinary printing and an appropriate toner layer is formed on the
developing roll at the beginning of developing by supplying a
sufficient amount of toner to the developing roll in case of a high
consistent printing so as to always obtain even images. With the
above method, a selective developing is prevented and toner
scattering and decrease of developing performance are prevented by
controlling a toner layer thickness to an optimum value so that a
color tandem type image forming apparatus giving excellent images
for a long time is provided.
[0079] Further, according to the present embodiment, sufficient
toner is supplied to a latent image on the latent image bearing
body without enlarging a distance between paper sheets by rotating
the developing roll 1.5 to 3 times as rapidly as the latent image
bearing body so that a rapid printing can be performed.
[0080] Further, according to the present embodiment, the developing
bias consisting of a direct current bias and an alternate current
bias superposed thereon is applied onto an electroconductive sleeve
on the most surface of the developing roll, the sleeve having a
volume resistivity of 10.sup.6 .OMEGA..multidot.cm.sup.3 or
smaller, so as to recover the toner layer on the developing roll to
the magnetic roll after developing whereby the bias of a direct
current bias and an alternate current bias superposed thereon acts
favorably to the gap between the rotating developing roll and
latent image bearing body, leading to obtaining a stable image for
a long time without decreasing an image consistency.
[0081] FIG. 7 and FIG. 8 illustrate the second embodiment of the
present invention. FIG. 7 is a table showing a state of generation
of ghost and staining of the developing roll comparing with the
case toner is exchanged and the case toner is not exchanged after
printing continuously 10,000 sheets of printing data of 5% of image
data. FIG. 8 is a table showing a state of consistency and
performance for following solid images comparing with the case
toner is exchanged and the case toner is not exchanged after
printing continuously 100 sheets of solid printing data of 100% of
image data.
[0082] As stated above, when high consistent printings, for
example, over 50% are successively performed, fine particles of
toner and stain of toner component adhere to the developing roll,
that is, so called toner filming occurs, which is apt to causes
unevenness of image such as mottle in image consistency owing to
the uneven toner thin layer 26 on the developing roll 22. Toner
performance for developing on the developing roll 22 decreases
sometimes when high consistent printings are successively performed
with a compact type developing apparatus. One possible reason is
selective developing of toner; another is deficiency of toner on
the developing roll. As a measure for selective developing, it is
proposed that toner is made return from developing roll 22 to the
magnetic roll 21 by reverse electric field after developing.
However the possibility of generating unevenness of image
consistency becomes large because of deficiency of toner on the
developing roll at the beginning of next developing, which is the
other possible reason.
[0083] Hence, in order to stabilize the image consistency of
successive printing: when printing data are transmitted, an image
data amount of each image is detected by a dot counter having an
unshown control device of the image forming apparatus; the toner
thin layer 26 is peeled off regularly after printing (between paper
sheets) from the developing roll 22 in accordance with a printing
coefficient determined by the data amount and the number of paper
sheets passed; and an interval of the action of recovering and
exchanging a toner layer on the developing roll 22 to the magnetic
roll 21 is variably set for refreshing. Though toner is usually
refreshed if toner is peeled off at every time after developing,
time for forming again a stable toner layer is necessary so that an
enough printing rate cannot be attained.
[0084] FIG. 7 and FIG. 8 show an effect of the case of peeling off
the toner thin layer 26 from the developing roll regularly in
accordance with a printing coefficient. FIG. 7 shows a state of
ghost and stain of the developing roll, variation of a toner charge
amount, and variation of a volume average particle size. A sample
of toner for toner particle size measurement is taken from the
toner on the developing roll. As for ghost and a level of staining
the developing roll: .largecircle. denotes the case that ghost part
38 shown in FIG. 3 or staining of the developing roll is not
observed; .DELTA. denotes the case that the phenomenon is observed
a little; and .times. denotes the case that the phenomenon is
apparently observed.
[0085] As shown in FIG. 7, if a toner layer is not exchanged in
case of the successive printing even when the image data of the
printing data is 5%, the toner charge increases so that a volume
average particle size as well as a consistency of the toner
reduces, which result in that the ghost and the staining of the
roll are apparently observed. On the contrary, if the toner layer
is changed after every 20 sheets printing, though the toner charge
is increased a little, a volume average particle size as well as a
consistency of the toner reduces little; consequently, the ghost
and the staining of the roll are not observed.
[0086] FIG. 8 shows a state of consistency and performance for
following solid images, variation of a toner charge amount, and
variation of a volume average particle size under the following
case: a case where the toner layer is not exchanged after the
successive printing of 100 sheets of the solid printing data having
an image data of 100%; a case where the toner layer is exchanged
after every 10 sheets printing; a case where the toner layer is
exchanged after every 10 sheets printing and when high consistent
printing data having an image consistency of 30% or more are
successively transmitted, a potential difference between the DC
bias (Vdc2) 28 of the magnetic roll 21 for example 350 V and the DC
(Vdc1) 27a of the developing roll increases by 50 V. In this
example, the DC bias (Vdc2) is 350 V, however, the bias may be
lower than the potential of the photosensitive body.
[0087] As shown in FIG. 8, when the printing data is 100%, the
performance for following solid images becomes faulty unless the
toner layer is not exchanged, the consistency is greatly decreased
and the loner charge amount as well as the toner volume average
particle size are also decreased. On the contrary, when the toner
layer is exchanged after every 10 sheets printing, though the toner
charge amount is decreased a little, the performance for following
solid images is good and the toner volume average particle size is
not decreased so greatly. And when the toner layer is exchanged
after every 10 sheets printing while a potential difference between
the DC bias (Vdc2) 28 of the magnetic roll 21 and the DC (Vdc1) 27a
of the developing roll is increased by 50 V, all the factors of the
consistency, the performance for following solid images, the toner
charge amount, and the volume average particle size are
excellent.
[0088] Consequently, when the toner layer on the developing roll 22
is not exchanged, ghost, staining of the developing roll and faulty
performance for following solid images are generated in any cases
of 5% and 100% solid images. The state is improved only by
exchanging the toner layer after every 20 sheets printing in case
of an image data of 5% and every 20 sheets printing in case of
solid (100%) images. Further, when a potential difference between
the DC bias (Vdc2) 28 of the magnetic roll 21 and the DC (Vdc1) 27a
of the developing roll is increased by 50 V, more excellent results
are obtained. A volume average particle size of the toner of 6.41
in case of after 100 sheets successive printing without exchange of
toner varies by a percentage change of 16% from the initial value
of 7.55. Seeing this value, it is thought that a volume average
particle size of the toner may to be controlled by exchanging the
toner layer.
[0089] Namely, when successive printing data are transmitted, a
data amount of individual image data is detected by a dot counter
and when a printing data amount of 5 pages or more of a data of 30%
or greater continues, a distance between sheets is controlled so as
to broaden by a distance, for example about 10 to 20 mm, in
accordance with a printing coefficient determined by the data
amount and the number of paper sheets passed, recovering the toner
thin layer 26 on the developing roll to the magnetic roll 21 and
replenishing the toner to the developing roll 22 after that.
[0090] Thus, the changing rates of the volume average particle size
of the toner thin layer 26 on the developing roll 22 is controlled
within 15% so that ghost, staining of the developing roll and
faulty performance for following solid images are prevented,
providing excellent images for a long time.
[0091] In order to exchange the toner thin layer 26 on the
developing roll 22, the toner thin layer 26 on the developing roll
22 is recovered to the magnetic brushes 30 by varying the DC bias
(V.sub.dc2) 28 in the state of applying the AC bias 27b after
developing. On the occasion of the toner exchange, the time period
for recovering and replenishing the toner layer on the developing
roll is adjusted by adjusting the distance between the paper sheets
in order to maintain an excellent printing rate. When the
rotational rate of the magnetic roll 21 is determined as one to two
times that of the developing roll 22, the exchange of the toner on
the developing roll is enhanced. It is preferable that the magnetic
roll 21 rotates in the same direction as the developing roll 22;
for example, when the developing roll 22 rotates counterclockwise,
the magnetic roll rotates also counterclockwise. If the ratio of
rotational rates of the developing roll and the magnetic roll is
unity, toner recovery from the developing roll is not enough. If
the ratio is 2 times or greater, the rotational rate of the
magnetic roll is increased, causing vibration or heat generation
and increasing stress to toner.
[0092] In order to supply sufficient toner onto the latent image on
the latent image bearing body without broadening the sheets
distance, rapid toner exchange is possible when the circumferential
rotational rate of the developing roll 22 is set as 1.1 times that
of the latent image bearing body. When the ratio is 1.1 or less, a
large sheets distance is required because time for beginning
developing is too long so that rapid printing cannot be
attained.
[0093] A saturated toner amount of the toner thin layer 26 on the
developing roll 22 is determined by the difference between the DC
bias (V.sub.dc2) 28 and the DC bias (V.sub.dc1) 27a. Hence, when
the DC bias (V.sub.dc2) 28 is set as 350 V and the DC bias
(V.sub.dc1) 27a as 100 V, the toner layer having a thickness of
approximately 1.0 mg/cm.sup.2 is obtained after a rotation of two
rounds of the developing roll. Though adjustment of the toner layer
can be done basically by the difference between the DC bias
(V.sub.dc2) 28 and the DC bias (V.sub.dc1) 27a, factors such as an
amount of toner charge and a magnetic strength of the developing
roll. Therefore, when successive printing is carried out, a
printing coefficient of the printing data is detected by a dot
counter provided in the control device of the image forming
apparatus and the DC bias (V.sub.dc2) 28 is varied in accordance
with the printing coefficient so as to obtain an even image. This
process is important as toner scattering from the developing roll
is prevented. An amount of toner on the developing roll is set as
comparatively small at ordinary printing so as to avoid scattering
and an amount of toner on the developing roll is adjusted only when
needed such as high consistent printing so as to form an optimum
toner layer at the beginning of developing. As explained in FIG. 8,
it is advantageous to set the difference between the DC bias
(V.sub.dc1) 28 and the DC bias (V.sub.dc1) 27a as a little
high.
[0094] In an tandem type image forming apparatus according to the
present invention having a hybrid developing apparatus constituted
as mentioned above, two-components developed material consisting of
toner 25 corresponding to an individual color such as yellow, cyan,
magenta or black and carrier 24 is supplied to the developing
apparatus 2 from the developing container 6, forming the magnetic
brushes 30 and charging the toner 25 by stir. The magnetic brushes
30 on the magnetic roll 21 is restricted with its layer thickness
by the restricting blade 29 and the thin layer of the only toner 25
is formed on the developing roll 22 by the potential difference
between the DC bias (V.sub.dc2) 28 applied to the magnetic roll 21
and the DC bias (V.sub.dc1) 27a, and AC bias 27b.
[0095] When the signal for beginning printing is transmitted from
an unshown control circuit, the latent image bearing body 3
comprising organic photosensitive material taking a positive charge
(positive OPC) is charged at first by the charging device 8, for
example to 400 V and after that, the latent image bearing body 3 is
exposed by LED which an exposing unit comprises and having a wave
length of 770 nm to a potential of 70 V after exposing to form a
latent image. The latent image is developed to form a toner image
with toner caused to jump to the latent image bearing body 3 from
the toner thin layer 26 on the developing roll 22 by the DC bias
(V.sub.dc1) 27a and AC bias 27b.
[0096] By the time a toner image is formed on the latent image
bearing body 3, a recording medium is taken from a cassette for
supplying paper and transported onto a transporting belt 5. A toner
image is transferred by applying a transfer bias by a transfer
device 9 disposed at the transfer position of individual color. The
toner image of each color is sequentially transferred to the
recording medium, which reaches a fixing device 10, is fixed, and
discharged.
[0097] The AC bias 27b applied to the developing roll 22 is a
square wave having a duty ratio of 50% or less. The period for
transporting toner 25 from the magnetic roll 21 to the developing
roll is shorter than the period for recovering toner 25 from the
developing roll 22 to the magnetic roll 21. Because the DC bias
(Vdc1) 27a and the AC bias 27b is applied directly to the
developing roll and the most surface of the developing roll 22 is
made of electro-conductive material having a volume resistivity of
10.sup.6 .OMEGA..multidot.cm.sup.3 or less, AC component of the
bias for recovering the toner thin layer 26 on the developing roll
to the magnetic roll 21 has a sharp peak so as to recover the toner
25 effectively. Therefore, the effectiveness of recovering toner
from developing roll 22 to the magnetic roll 21, which is a
transporter for developer material, is enhanced so that staining of
the developer roll with toner is prevented and recovery of toner
from the latent image bearing body 3 toward the developing roll is
effectively performed, which result in providing a developing
apparatus capable of obtaining quality images having long stability
and preventing generation of fogging.
[0098] Further, according to the present invention, when an unshown
control circuit receives printing data, an image data amount of
each image is detected by a dot counter having an unshown control
device of the image forming apparatus; the toner thin layer 26 is
regularly peeled off at an interval in accordance with a printing
coefficient determined by the data amount and the number of paper
sheets passed so as to refresh toner by recovering and exchanging
the toner layer on the developing roll 22 to the magnetic roll 21.
The refreshing is carried out in such a manner that the toner thin
layer 26 on the developing roll 22 is recovered to the magnetic
brushes 30 at a period between the sheets distance after developing
by varying the DC bias (V.sub.dc2) 28 in the state of applying the
AC bias 27b. On the occasion of the toner exchange, the time period
for recovering and replenishing the toner layer on the developing
roll is adjusted by adjusting the distance between the paper sheets
in order to maintain an excellent printing rate. When thus the
percentage change of a volume resistivity of the toner thin layer
26 on the developing roll 22 is controlled within 15% and the
amount of toner charge is controlled within the range of between 5
and 20 .mu.C/g, a rapid, inexpensive and compact tandem-type image
forming apparatus can be provided having a compact hybrid-type
developing apparatus which gives an excellent images whose image
quality is stable for a long time, preventing from generation of
ghost, staining the developing roll and faulty performance for
following solid images.
[0099] A voltage value of the DC bias (V.sub.dc1) 27a, the AC bias
27b or the DC bias 28, V.sub.pp or the frequency that has been
explained above is an example and is obviously variable according
to the situation.
[0100] Therefore, according to the second embodiment of the present
invention, by variably setting an interval of the action of
recovering a toner layer on the developing roll to the magnetic
roll after printing (between paper sheets) in accordance with a
printing coefficient determined by the data amount and the number
of paper sheets passed, toner filming caused by sticking fine
particles or contamination of toner constituents is prevented even
when images having consistency over 50% are successively printed.
The unevenness of images such as generation of heterogeneous image
consistency or unevenness of the toner layer caused by the toner
filming can be avoided. When images with high consistency are
successively printed, poor toner charge caused by selective
developing of toner or deficiency of toner amount on the developing
roll occurs so as to diminish toner performance regarding
developing. Even on this occasion, an appropriate amount of toner
can constantly be supplied onto the developing roll so that
generation of heterogeneous image consistency can be avoided while
keeping enough printing rate. Therefore, excellent images can be
obtained for a long time, preventing from generation of ghost, from
staining of the developing roll or from poor performance for
following solid images.
[0101] In this case, a rotational rate of the developing roll is
1.1 times or more that of the latent image bearing body so that
enough toner is supplied to a latent image on the latent image
bearing body without broadening the distance between paper sheets,
printing rapidly; and a rotational rate of the developing roll is
1-2 times that of the magnetic roll so that toner exchanging on the
developing roll is enhanced, avoiding stress to toner caused by
insufficient recovery of toner, vibration or heat generation.
[0102] Further, the changing rates of the volume average particle
size of the toner thin layer 26 on the developing roll 22 is
controlled at first within 15% by recovering and exchanging the
toner layer on the developing roll to the magnetic roll so that
unevenness of image consistency, ghost, staining of the developing
roll and faulty performance for following solid images are
prevented while a sufficient printing rate is maintained, providing
a developing method in an image forming apparatus which gives
excellent images for a long time.
[0103] Even if a printing coefficient varies, uniform images are
obtained by changing a toner amount on the developing roll within
the range from 0.5 to 1.7 mg/cm.sup.2. For example, in ordinary
printing, toner scattering is diminished by setting a toner amount
on the developing roll as comparatively small; in high consistency
printing, an appropriate toner layer at the beginning of developing
can be formed by supplying an enough amount of toner to the
developing roll so that uniform images are always obtained.
[0104] Further, selective developing is avoided by controlling an
amount of toner charge on the developing roll at the beginning of
developing with the range from 5 to 20 .mu.C/g, and toner
scattering and diminishing of toner performance regarding
developing are avoided by controlling a toner layer thickness to an
optimum value, leading to obtaining excellent images for a long
time.
[0105] Further, the surface of the developing roll consists of an
approximately uniform electro conductive sleeve having a
conductivity of 10.sup.6 .OMEGA..multidot.cm.sup.3 or under to
which a bias is applied. Thus, the bias applied to the developing
roll acts favorably to the gap between the rotating developing roll
and the latent image bearing body and the magnetic roll so that
image consistency does not decrease, leading to obtaining excellent
images for a long time.
* * * * *